1. Field of the Invention
The present invention relates generally to emergency lighting systems for energizing a lamp when normally available AC power is disrupted, and more particularly to emergency lighting systems for use in combination with fluorescent or other vapor lamps designed to operate in combination with a ballast.
2. Description of Prior Art
The most simple emergency lighting systems employ a trickle battery charger, a battery, a switch, and a low voltage incandescent lamp. The trickle battery charger is connected to the AC line and supplies a controlled, small charging current to the battery. When power fails on the AC line, the switch automatically connects the battery to the low voltage incandescent lamp, lighting the lamp.
One variation of this simple system is to include a means for stepping up the voltage from the battery so that the emergency light can be an incandescent lamp of standard voltage. If this is done, then by suitable automatic switching the very same incandescent lamp can be lit all of the time, normally from the AC line, but when AC power fails, then from battery power by way of the means for stepping up the voltage. More commonly, the AC line voltage is stepped down to the battery voltage, and one incandescent lamp designed for the battery voltage is used both in normal AC operation and battery powered.
In those buildings most likely to need emergency lighting systems, or to be required to have such by local ordinances, most of the lamps in use are fluorescents. Unlike incandescent lamps, fluorescent or other vapor lamps require a ballast. This may be external in the fixture, or the lamp may be internally ballasted, having the ballast incorporated within the lamp as, for example, a resistor in the lamp base. Thus, fluorescent fixtures contain a ballast to control the operating beam current of the lamp. Some fluorescent fixtures also include a starting capacitor to assist ignition of the lamp.
As noted above, conventional emergency lighting systems include a rechargeable battery and circuit means for charging the battery while the AC line is operative. The charging circuits in the conventional systems are themselves powered by the AC line to develop the requisite battery charging current. An example of such system is disclosed in U.S. Pat. No. 3,684,891, issued Aug. 15, 1972. The patented system shows a charging circuit connected across the AC main, a rechargeable battery and an inverter oscillator having two output leads each of which must be connected to terminals at both ends of a fluorescent lamp, respectively.
It will be understood that if the above prior art system is connected to the lamp while the lamp remains inserted within a fluorescent fixture for normal operation, a problem will arise in the event the ballast which energizes the lamp includes a starting capacitor connected across the leads from the ballast running to the terminals on the fixture into which the lamp is inserted. This is so because the starting capacitor, which has a relativey high impedance at the normal AC operating frequency of 60 hertz, acts as a short circuit across the output of the inverter oscillator of the patented system, the inverter oscillator producing an AC potential at a frequency of 10 kilohertz (10,000 hertz), according to the patent.
Thus, as cautioned in the disclosure of the patented system, the system may be used to provide emergency power for only two out of the four fluorescent lamps contained in a four lamp fixture using conventional AC ballasts. Therefore, no more than half of the normal illumination available from each fluorescent lamp fixture can be obtained when using the patented system.
A further problem can arise with emergency lighting systems in which electrical connections are made between the system and the lamp while the lamp remains mounted in its own fixture. For example, in the above patented system, a separate connection must be provided to the battery charging circuit from the AC main. Such wiring to the AC main, and any additional wiring required to connect with the ballast leads, often requires a licensed electrician to certify the rewiring for compliance with applicable building codes, thereby leading to installation costs which may easily exceed the cost of the entire emergency system itself.
The above and other shortcomings in the prior art emergency lighting systems are overcome by the present invention.